Eva-Maria Schötz

Quantitative differences in tissue surface tension influence zebrafish germ layer positioning

This study provides direct functional evidence that differential adhesion, measurable as quantitative differences in tissue surface tension, influences spatial positioning between zebrafish germ layer tissues. We show that embryonic ectodermal and mesendodermal tissues generated by mRNA‐overexpression behave on long‐time scales like immiscible fluids. When mixed in hanging drop culture, their cells segregate into discrete phases with ectoderm adopting an internal position relative to the mesendoderm. The position adopted directly correlates with differences in tissue surface tension. We also show that germ layer tissues from untreated embryos, when extirpated and placed in culture, adopt a configuration similar to those of their mRNA‐overexpressing counterparts. Down‐regulating E‐cadherin expression in the ectoderm leads to reduced surface tension and results in phase reversal with E‐cadherin‐depleted ectoderm cells now adopting an external position relative to the mesendoderm. These results show that in vitro cell sorting of zebrafish mesendoderm and ectoderm tissues is specified by tissue interfacial tensions. We perform a mathematical analysis indicating that tissue interfacial tension between actively motile cells contributes to the spatial organization and dynamics of these zebrafish germ layers in vivo.

Glassy dynamics in three-dimensional embryonic tissues

Many biological tissues are viscoelastic, behaving as elastic solids on short timescales and fluids on long timescales. This collective mechanical behaviour enables and helps to guide pattern formation and tissue layering. Here, we investigate the mechanical properties of three-dimensional tissue explants from zebrafish embryos by analysing individual cell tracks and macroscopic mechanical response. We find that the cell dynamics inside the tissue exhibit features of supercooled fluids, including subdiffusive trajectories and signatures of caging behaviour. We develop a minimal, three-parameter mechanical model for these dynamics, which we calibrate using only information about cell tracks. This model generates predictions about the macroscopic bulk response of the tissue (with no fit parameters) that are verified experimentally, providing a strong validation of the model. The best-fit model parameters indicate that although the tissue is fluid-like, it is close to a glass transition, suggesting that small changes to single-cell parameters could generate a significant change in the viscoelastic properties of the tissue. These results provide a robust framework for quantifying and modelling mechanically driven pattern formation in tissues.

Quantitative characterization of planarian wild-type behavior as a platform for screening locomotion phenotypes

SUMMARY Changes in animal behavior resulting from genetic or chemical intervention are frequently used for phenotype characterizations. The majority of these studies are qualitative in nature, especially in systems that go beyond the classical model organisms. Here, we introduce a quantitative method to characterize behavior in the freshwater planarian Schmidtea mediterranea. Wild-type locomotion in confinement was quantified using a wide set of parameters, and the influences of intrinsic intra-worm versus inter-worm variability on our measurements was studied. We also examined the effect of substrate, confinement geometry and the interactions with the boundary on planarian behavior. The method is based on a simple experimental setup, using automated center-of-mass tracking and image analysis, making it an easily implemented alternative to current methods for screening planarian locomotion phenotypes. As a proof of principle, two drug-induced behavioral phenotypes were generated to show the capacity of this method.

SAPling: a Scan-Add-Print barcoding database system to label and track asexual organisms.

SUMMARY We have developed a ‘Scan-Add-Print’ database system, SAPling, to track and monitor asexually reproducing organisms. Using barcodes to uniquely identify each animal, we can record information on the life of the individual in a computerized database containing its entire family tree. SAPling has enabled us to carry out large-scale population dynamics experiments with thousands of planarians and keep track of each individual. The database stores information such as family connections, birth date, division date and generation. We show that SAPling can be easily adapted to other asexually reproducing organisms and has a strong potential for use in large-scale and/or long-term population and senescence studies as well as studies of clonal diversity. The software is platform-independent, designed for reliability and ease of use, and provided open source from our webpage to allow project-specific customization.